The present invention relates to the extrusion of thermoplastic polymeric materials, and, more particularly, to an improved extrusion apparatus for producing highly oriented fibers and other extrudates of thermoplastic polymeric material.
In the production of thermoplastic polymeric fibers, it is often desirable to develop as high a degree of molecular orientation in the fiber as possible in order to optimize the strength and other physical properties of the fiber. Present day commercial production of such fibers usually involves the steps of melt or solution spinning, followed by a drawing and/or annealing operation for the purpose of developing at least some degree of such orientation in the fiber. However, the degree of orientation that has generally been achievable by such methods leaves substantial room for improvement.
Theoretically, the ideal crystalline structure for optimizing the strength and other physical properties of such fibers would be one composed substantially entirely of "extended chain crystals", i.e., crystals wherein each polymer chain extends in a completely straight line parallel to the fiber axis. The extended chain crystal is a near perfect form of crystal and is believed to have very desirable physical properties. A number of investigators have reported successfully producing extended chain crystals on a laboratory scale employing various techniques such as shearinduced crystallization, pressure-induced crystallization, and solid state extrusion.
From the standpoint of improved fiber production, the solid state extrusion technique for obtaining extended chain crystals has created a considerable amount of interest. The most promising development along these lines thus far has been the work of Southern et al, Apply Poly Sci., v. 14, 2305-2317 (September, 1970); DIE Makromol Chem. Band 162, 1972, Siete 19-30; who reported the solid state extrusion of polyethylene in a capillary rheometer. The fibers produced using their process are highly oriented with unusually high strength; and because they consist of essentially continuous extended chain crystals, the fibers are transparent. This process, however, has two major disadvantages which make it impractical for the commercial production of extended chain crystal fibers. First of all, the process requires extremely high pressures of at least 2000 atmospheres, which is much higher than can be achieved in commercial extruders. Secondly, the process results in very slow formation rates on the order of only a few inches an hour.
The Southern et al method represents an attempt to induce orientation of the polymer chains in the molten polymeric material by means of elongational or extensional flow of the polymer melt into the extrusion die, and to maintain that orientation by subsequent crystallization of the polymer. The problems encountered by Southern et al apparently were due to a premature crystallization of the polymer before it had a chance to enter the extrusion die, thereby producing a plugging effect in the entrance region of the extrusion die. In essence, subsequent extrusion through the die of the plug formed in the entrance region of the die would be somewhat analogous to forcing a plastic golf tee through a cylinder with an inside diameter roughly equivalent to the outside diameter of the shank of the tee. High extrusion forces would be required to deform the golf tee head and extrude it through the cylinder. Likewise, a plugging effect in the entrance region of the die would necessitate the extremely high pressures employed by Southern et al to solid state extrude the crystallized polymer through the die, and would also be responsible for the very slow production rate of the resulting extrudate.
The above-postulated explanation for the difficulty encountered in the Southern et al process becomes particularly plausible when considering the various thermodynamic phenomena which occur in a molten polymeric material undergoing crystallization under elongational or extensional flow conditions such as are developed in the entrance region of the die. Under such flow conditions, the velocity gradient in the direction of flow becomes rather high, and such increasing velocity gradient produces several different effects. First of all, the effective melting point of the polymeric material, i.e., the temperature controlling the point at which crystallization of the polymer melt will be initiated, becomes significantly elevated above the normal melting point of the polymeric material, i.e., the quiescent atmospheric pressure melting point. Secondly, the free energy change upon crystallizaton, i.e., the thermodynamic driving force for crystallization, is also increased. Thirdly, the forces developed by the increasing velocity gradient cause the polymer chains to be extended and to develop parallel alignment, thereby dramatically reducing the major resistance to crystallization. With the effective melting point of the polymeric material becoming elevated, the driving force for crystallization increasing, and the resistance to crystallization decreasing, the interaction of all these effects brings about a condition which is highly conducive to extremely rapid crystallization.
In the method described by Southern et al, the entrance region of the extrusion die was maintained at a temperature which was above the normal melting point of the polymeric material, but apparently below the elevated effective melting point for the flowing polymer. Under such conditions, the above-described thermodynamic considerations would make it very likely for crystallization of the polymeric material to be prematurely induced in the entrance region of the die, thereby bringing about in the entrance region of the die the plugging effect discussed above.